Question
The octahedral complex of a metal ion $${M^{3 + }}$$ with four monodentate ligands $${L_1},{L_2},{L_3}$$ and $${L_4}$$ absorb wavelengths in the region of red, green, yellow and blue, respectively. The increasing order of ligand strength of the four ligands is:
A.
$${L_4} < {L_3} < {L_2} < {L_1}$$
B.
$${L_1} < {L_3} < {L_2} < {L_4}$$
C.
$${L_3} < {L_2} < {L_4} < {L_1}$$
D.
$${L_1} < {L_2} < {L_4} < {L_3}$$
Answer :
$${L_1} < {L_3} < {L_2} < {L_4}$$
Solution :
For a given metal ion, weak field ligands create a complex with smaller $$\vartriangle ,$$ which will absorbs light of longer $$\lambda $$ and thus lower frequency. Conservely, stronger field ligands create a larger $$\vartriangle ,$$ absorb light of shorter $$\lambda $$ and thus higher vi.e. higher energy.
$$\mathop {{\text{Red}}}\limits_{\lambda = 650nm} < \mathop {{\text{Yellow}}}\limits_{570\,nm} < \mathop {{\text{Green}}}\limits_{490\,nm} < \mathop {{\text{Blue}}}\limits_{450\,nm} $$
So order of ligand strength is
$${L_1} < {L_3} < {L_2} < {L_4}$$
For a given metal ion, weak field ligands create a complex with smaller $$\vartriangle ,$$ which will absorbs light of longer $$\lambda $$ and thus lower frequency. Conservely, stronger field ligands create a larger $$\vartriangle ,$$ absorb light of shorter $$\lambda $$ and thus higher vi.e. higher energy.
$$\mathop {{\text{Red}}}\limits_{\lambda = 650nm} < \mathop {{\text{Yellow}}}\limits_{570\,nm} < \mathop {{\text{Green}}}\limits_{490\,nm} < \mathop {{\text{Blue}}}\limits_{450\,nm} $$
So order of ligand strength is
$${L_1} < {L_3} < {L_2} < {L_4}$$